With technical improvement in recent years, liquid crystal displays (LCDs), plasma display panels (PDPs), electroluminescence displays (ELDs), etc. have been developed in addition to conventional cathode ray tubes (CRTs) as image displays and have been used practically. As LCDs have been technically improved to provide wide viewing angles, high resolution, high response, good color reproduction, and the like, applications of LCDs are spreading from laptop personal computers and monitors to television sets. In a basic LCD structure, a pair of flat glass substrates each provided with a transparent electrode are opposed via a spacer to form a constant gap, between which a liquid crystal material is placed and sealed to form a liquid crystal cell, and a polarizing plate is formed on the outside surface of each of the pair of glass substrates. In a conventional technique, a glass or plastic cover plate is attached to the surface of the liquid crystal cell in order to prevent scratches on the polarizing plate bonded to the surface of the liquid crystal cell. However, the placement of such a cover plate is disadvantageous in terms of cost and weight. Thus, a hard coating process has gradually been used to treat the surface of polarizing plates.
For the hard coating process, a hard-coated film is generally used in which a thin hard-coating layer with a thickness of 2 to 3 μm has been formed on one or both surfaces of a transparent plastic film substrate. Generally, the hard-coating layer is formed using resins for forming a hard-coating layer such as thermosetting resins or ultraviolet(UV)-curable resins. If such resins are applied to a glass plate to form the hard-coating layer, it can exhibit a pencil hardness of 4H or more. If a hard-coating layer with an insufficient thickness is formed on a transparent plastic film substrate, however, the pencil hardness of the layer can be generally affected by the substrate and reduced to 3H or less.
LCD applications have come to include home television sets, and thus it is easily expected that the users of general home television sets should handle LCD television sets in the same manner as in the case of conventional glass CRT television sets. Glass CRTs have a pencil hardness of about 9H. Thus, hard-coated films to be used for LCDs have been required to have higher hardness.
An increase in the hardness of hard-coated films is possible by increasing the thickness of their hard-coating layer. However, such an increase in thickness causes cracking of the hard-coating layer or curling due to hardening and shrinking of the hard-coating layer. In order to overcome these problems, techniques have been proposed, as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 09-113728, 11-300873, 2000-52472, and 07-287102.
JP-A No. 09-113728 discloses a protective film for polarizing plates, which includes a transparent resin film and a coating layer formed on at least one surface of the transparent resin film by curing a composition containing a UV-curable polyol (meth)acrylate resin. It also discloses that the UV-curable polyol (meth)acrylate resin may be dipentaerythritol acrylate. According to JP-A No. 09-113728, the cured coating layer with a thickness of 10 μm or more can provide a pencil hardness of 4H or more, if it is formed of a resin whose main component is dipentaerythritol acrylate. It also discloses that if an epoxy resin is additionally used, curling due to hardening and shrinking can be reduced. According to the invention as disclosed in JP-A No. 09-113728, however, it is difficult to sufficiently reduce curling.
JP-A No. 11-300873 discloses a hard-coated film, which includes a film of a plastic base material, a buffer monolayer or multilayer with a thickness of 3 to 50 μm formed on at least one surface of the film, and a hard-coating layer with a thickness of 3 to 15 μm formed on the buffer layer. JP-A No. 11-300873 also discloses that the hard-coated film as a whole has a pencil hardness of 4H to 8H. However, when the two-layer structure is employed that is composed of the buffer layer and the hard-coating layer, there is a problem in that the number of manufacturing steps increases, and thereby the production efficiency decreases.
JP-A No. 2000-52472 discloses a hard-coated film including a substrate and a cured resin coating layer that is formed by a process including the steps of forming, on the substrate, a first hard-coating layer of a cured resin that contains inner cross-linked inorganic or organic ultrafine particles and then forming a second hard-coating layer of a thin clear cured resin film that is free of inner cross-linked inorganic or organic ultrafine particles. Like the invention as disclosed in JP-A No. 11-300873, however, when the two-layer structure including the first and second hard-coating layers is employed for the cured resin coating layer, there also is a problem in that the number of manufacturing steps increases, and thereby the production efficiency decreases.
JP-A No. 07-287102 discloses an antireflection film including a transparent substrate film and a hard-coating layer and a low-refractive-index layer that are formed on at least one of the front and back surfaces of the transparent substrate film. It also discloses that solvent-drying type resins may be used as a material for the hard-coating layer. If a reactive group-free polymer is added, hardening and shrinking that occur when ionizing radiation-curable resins or the like are cured are prevented and thereby curling can be effectively reduced. However, the addition of such a polymer has a problem in which sufficiently high surface hardness is difficult to obtain.